For years, fish fossils have turned up in the deserts of western Egypt, and nobody could explain why. Now we know: Long ago, the Egyptian desert was home to a body of water the size of Lake Michigan.

As the home to one of the world's most ancient civilizations, the Egyptian environment can seem almost eternal, with the Nile River forever providing the only fertile refuge from the endless expanse of desert on both sides. But once, long ago, the southwestern deserts were home to a massive lake that stretched some 250 miles away from the Nile. The existence of such a lake explains a few longstanding mysteries, but it also raises a few new questions.

The biggest puzzle it solves is just how a bunch of fish fossils, clearly related to those found in the Nile, were discovered out in the open desert. The lake provides these fish with a place to live, and the existence of the lake also neatly explains the fish's origins. After all, the huge amounts of water needed for such a lake must have come from somewhere, and the obvious source is the Nile itself. A pass known as Wadi Tushka could have connected the Nile with this lake, provided the Nile's annual flooding was more severe than it is today.

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Beyond the presence of the fish fossils, the ancient lake is also known from radar maps that hint at the ancient topography of the region, as well as a few archaeological sites apparently placed on the shorelines of the lake in more modern times. All this puts together a decently compelling case, but the Egyptian lake is missing a few crucial pieces of supporting evidence that really ought to be there.

For one thing, most vanished lakes leave behind geological evidence of their old shorelines - these marks are rather colorfully known as bathtub rings. The western United States is full of such bathtub rings that offer definitive proof of lost mega-lakes. But whatever bathtub rings the Egyptian lake left behind have long since been blasted away by millennia of shifting sands. There's also no sedimentary evidence for the lake. That doesn't disprove the existence of the lake, but it makes it an awful lot harder to prove that it exists.

We also don't actually know for sure how the lake developed. Though the Nile seems like the obvious source, it's not the only possibility. Indeed, a few ancient sedimentary remnants from springs and archaeological sites suggest the western deserts were once home to a bunch of small lakes created by rain and groundwater. Over time, these lakes might have grown bigger and ultimately merged into the one mega-lake, which in turn started flowing into the Nile. That would have then allowed the Nile fish to enter the lake.

So then, we know there was once a lake in Egypt, which is a pretty cool finding. But exactly how it formed, and how it affected the ecology of the Nile and surrounding deserts, not to mention any impact it had on the human populations of ancient Egypt, are all things we still have to figure out.